1. Field of the Invention
Embodiments of the invention relate to an organic light emitting diode (OLED) display capable of adjusting a high potential driving voltage applied to pixels by a monitoring feedback method.
2. Discussion of the Related Art
Various flat panel displays whose weight and size are smaller than cathode ray tubes have been recently developed. Examples of the flat panel displays include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and an electroluminescence device.
Because the PDP has a simple structure and is manufactured through a simple process, the PDP has been considered as a display device having characteristics such as lightness in weight and thin profile and providing the large-sized screen. However, the PDP has disadvantages such as low light emitting efficiency, low luminance, and high power consumption. A thin film transistor (TFT) LCD using a TFT as a switching element is the most widely used flat panel display. However, because the TFT LCD is not a self-emission display, the TFT LCD has a narrow viewing angle and a low response speed. The electroluminescence device is classified into an inorganic light emitting diode display and an organic light emitting diode (OLED) display depending on a material of an emitting layer. In particular, the OLED display has characteristics such as a fast response speed, a high light emitting efficiency, a high luminance, and a wide viewing angle because the OLED display is a self-emission display.
The OLED display, as shown in FIG. 1, includes an organic light emitting diode. The organic light emitting diode includes organic compound layers between an anode electrode and a cathode electrode.
The organic compound layers include a hole injection layer HIL, a hole transport layer HTL, an emitting layer EML, an electron transport layer ETL, and an electron injection layer EIL.
When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emitting layer EML and form an exciton. Hence, the emitting layer EML generates visible light.
In the OLED display, pixels each including the above-mentioned organic light emitting diode are arranged in a matrix format, and a brightness of the pixels selected by a scan pulse is controlled depending on a gray level of video data. In the OLED display, the pixel is selected by selectively turning on a TFT used as an active element and remains in a light emitting state by a voltage charged to a storage capacitor.
The OLED display is driven by a digital method or an analog method. The digital method displays a gray scale according to intensity of data voltage or data current applied to pixels. On the other hand, the analog method displays the gray scale according to a supplying time of data voltage or data current applied to pixels in a constant intensity. The OLED display adopting the analog method cannot display a gray scale correctly because electrical characteristics (a threshold value, an electron mobility and so on) of a driving thin film transistor (TFT) are changed in each pixel depending on the intensity of data voltage or data current applied to the pixel. Herein, the driving TFT controls amount of current flowing through the OLED depending to the intensity of data voltage or data current applied to the pixel. However, the OLED display adopting the digital method can display a correct gray scale because the driving TFT is used as only a switching element. In recent, there have been many techniques for driving the OLED display by the digital method.
In general, the OLED display adopting the digital method uses a monitoring feedback method in order to compensate a deterioration of picture quality that is generated due to a characteristic variation of organic material contained in the OLED by a change of external temperature. Referring to FIG. 2, the monitoring feedback method includes steps of forming a pixel monitoring part MP at one side of a display panel in order to predict a deterioration degree of the pixels, sampling a voltage which is feed-backed after applying a constant monitoring current to the pixel monitoring part MP, and adjusting a high potential driving voltage applied to the pixels based on the sampled voltage. The pixel monitoring part MP includes a R (red) monitoring OLED MR to which a first monitoring current Ir is applied, a G (green) monitoring OLED MG to which a second monitoring current Ig is applied, and a B (blue) monitoring OLED MB to which a third monitoring current Ib is applied. If the external temperature is changed, the characteristic of the organic material contained the OLEDs MR, MG and MB is changed. Thus, resistance components of the OLEDs MR, MG and MB are changed. As a result, feedback voltages Vrf, Vgf and Vbf having the changed voltage levels are supplied to a power IC. The power IC adjusts a first high potential driving voltage VOR supplied to the R pixel of the display panel using the R feedback voltage Vrf, adjusts a second high potential driving voltage VOG supplied to the G pixel of the display panel using the G feedback voltage Vgf, and adjusts a third high potential driving voltage VOB supplied to the B pixel of the display panel using the B feedback voltage Vbf. The more resistance components of the OLEDs MR, MG and Mb increase, the more feedback voltages Vrf, Vgf and Vbf increase. In general, if the external temperature is lower, amount of the current flowing through the pixels is decreased, thereby lowering brightness. In order to compensate for the lowered brightness, the high potential driving voltages VOR, VOG and VOB are increased gradually using the feedback voltages Vrf, Vgf and Vbf, respectively, as shown in FIG. 3.
However, in the OLED display adopting the monitoring feedback method, there is a problem demanding an output voltage beyond the maximum output voltage of the power IC as shown in FIG. 3 because the more external temperature is lower, the more output voltages VOR, VOG and VOB are gradually increased in order to prevent the lower of brightness by a change of the external temperature. In the case of demanding an output voltage power beyond the maximum output voltage of the power IC, there is a defect of picture quality such as a flickering phenomenon because the output voltage of the power IC is unstable.